Vegetation changes could diminish US wildfires

Sean Parks at the US Forest Service’s Rocky Mountain Research Station and colleagues used a computer model to show that the severity of western US wildfires will probably decline in a warming climate. But they believe that trend is contingent on the vegetation itself changing with the climate – a balance that could be dangerous to interfere with.

"Resisting climate-induced changes to vegetation and disturbance regimes will likely be counter-productive in the long run, particularly in the forested regions of the western US," said Parks. "Allowing climate-induced changes to occur, or even facilitating change, with the use of managed fire and restoration treatments, is a more sustainable approach."

Scientists are keen to understand how wildfires will occur under climate change – not just because of the danger they can pose to humans, but because of their feedback to the ecosystem and climate, for example by releasing huge amounts of carbon. Aspects such as the area burned and the length of the wildfire season have been well-studied, but less attention has been paid to fire severity, a parameter that defines how much a wildfire alters a landscape for a certain amount of time.

Parks and colleagues drew on statistics derived from satellite imagery for wildfire severity in the western US and meteorological data to build a model linking fire and climate. By feeding into this model, climatic projections based on the RCP 8.5 scenario of increasing greenhouse gas emissions, the researchers then made predictions of how fire severity will be affected by climate change.

The results were dependent on location, but in general there was a decrease in fire severity, with the largest reductions tending to coincide with areas of cool, high elevation, wet forests, such as the Greater Yellowstone Ecosystem that straddles the states of Montana, Wyoming and Idaho.

Yet Parks and colleagues point out that their model implicitly takes into account changes in vegetation that will occur under climate change – and different vegetation has different characteristics when it comes to wildfire severity. "Simply put," said Parks, "because plant species distributions should shift in response to climate change, fire regime characteristics will also shift."

Shifts in species distributions are often "catalyzed", or stimulated, by natural disturbance events such as fire. "It is these disturbance events that often allow sites to be colonized by new species that are more suited to the emerging climate," said Parks. "If we, as humans, do not allow natural disturbances to occur" – by extinguishing lightning-sparked wildfires, for example – "then species shifts will likely be slower than they would otherwise."

Now Parks wants to explore other ways to characterize changes to wildfire regimes under climate change.